High-voltage apparatus having an insulation of foamed material



Sept. 2, 1969 w. BERENDS 3,465,154

HIGHVOLTAGE APPARATUS HAVING AN INSULATION 0F FOAMBD MATERIAL Filed March 9, 1967 2 Sheets-Sheet 2 INVENTOR. WERNER BERENDS BY w m AG T I Int. Cl. Gin 1/00 US. Cl. 250-108 12 Claims ABSTRACT OF THE DISCLOSURE A high-voltage electrical apparatus particularly an X-ray apparatus having an electrical screen. The parts of the apparatus which are at a high potential relative to the screen are embedded in a foamed insulating material.

The invention relates to an electrically screened highvoltage apparatus, in particular an X-ray apparatus.

It is already known to incorporate the parts of highvoltage apparatus which are at a high voltage in an oil filled grounded metal housing. The oil may not contain any water since water considerably reduces the breakdown strength of the oil; therefore the housing must be sealed as completely as possible in order to prevent moist air from penetrating and water from being absorbed by the oil which, however, cannot be achieved entirely owing to the diffusion through the rubber diaphragm of the expansion vessel which must be present because the oil, on being heated, expands in volume. In addition the insulating properties of the oil deteriorate in the long run as a result of resinification or formation of fibre bridges.

In addition it known to use in the above described apparatus gaseous sulphurhexafluoride (SP as an insulating material. In this case the housing must be even better sealed than when oil is used since the gas is at excessive pressure; in this case the gas pressure must be checked continuously. When SP is used as an insulating material in X-ray apparatus, the additional drawback occurs that the gas is decomposed by the X-rays, highly reactive decomposition products being formed which attack or etch the remaining insulating materials and the glass of the X-ray tube, respectively. The extent to which the corrosion phenomena occur depends upon various factors, for example, upon the type of voltage at the X-ray tube (alternating voltage, direct voltage), upon the quantity and the hardness of the X-rays, upon the irradiated gas volume, and so on. Finally it is known, in particular in X-ray apparatus, to fill the free spacebetween the housing and the X-ray tube with an epoxy resin which is then hardened by suitable chemical additions. However, the epoxy resin shrinks on hardening so that the tube is destroyed if prior to filling it is not provided with a lining which absorbs the pressure and which consists, for example, of silicon rubber. In addition to the high cost and the comparatively large increase in weight, this solution suffers from the drawback that the X-rays are considerably attenuated by the insulating material. In order to avoid such an attenuation of the radiation intensity the tube must, at the place at which the radiation window is situated, comprises a neck or the like throough which the radiation can directly emerge without having to pass the insulating material. However, this means that the glass tube cannot be constructed to be rotationally symmetrical so that the manufacture becomes more expensive accordingly.

The above described known solutions of insulating such parts which are at a high voltage have a further drawback United States Patent when used in X-ray radiating apparatus, namely that the fragile X-ray tube is insufficiently protected from heavier shocks. Therefore shock-absorbing means must be provided inside or outside the radiator so that the required number of structural elements is increased again.

According to the invention the above drawbacks are avoided in that the parts which are at high voltage are embedded wholly or partly in a foamed material. As foamed materials are to considered, for example, phenol resin foam, polystyrol foam or polyurethane foam.

Preformed bodies from foamed material may be used and the free space between the structural elements and housing may be filled with a liquid or gaseous insulating material. This possibility will be used in particular when the housing is already constructed for liquid or gaseous insulating materials. The penetration of the gas or the liquid into the foamed material maybe prevented by using a closed-celled material or by providing a layer of lacquer on the foamed material which prevents the penetration of the gas or the liquid. It is preferable to metallize the surface of the body of foamed material Which engages the housing to prevent the occurrence of electric charges and in addition it is useful to undulate the non-metallized surface in order to lengthen the surface-leakage bath.

Alternatively the whole free space between the structural elements and the housing may be filled with a foamed material. The freedom of movement of the structural elements relative to the housing then restricted to such an extent that insulators from casting resin or the like which are otherwise necessary for supporting the structural elements, in particular the elements conducting the highvoltage, become superfluous. The insulators may be replaced by a foamed material of a suitable density and also the sockets for receiving the high-voltage plugs or the like may be constructed from the material.

Finally the whole housing also may be embedded in a resilient foamed material so that instead of the comparatively thick-Walled housing only a thin-walled electric screening is necessary which, in X-ray apparatus, preferably consists of lead for safeguarding against the radiation and which is provided in the resilient foamed material of the envelope.

In order that the invention may readily be carried into effect one embodiment of an X-ray apparatus will now be described in greater detail, by way of example, with reference to FIGURES 1 to 3.

FIGURE 1 shows an X-ray tube 1 comprising an anode 2 and a cathode 3; the axis of the electron beam is denoted by 9. The tube 1 is arranged in a housing 5 which is provided with a radiation emerging window 10 and which is closed at the ends by flanges '6 which are secured to the housing by means of screws 7. An insulating gas, for example, the sulphurhexafluoride may be supplied or conducted away through the filling apertures 8. The tube 1 is embedded in a preformed body 11 of foamed material and may also be glued to said body by means of a layer 16 of epoxy resin provided on the glass wall. Further details which are not essential for the invention are not shown to avoid complexity of the drawing.

As foamed materials may be used, for example, polystyrol foam, phenol resin foam or polyurethane foam since these foams have the advantage, in addition to other favourable properties, that the hardness, cellular construction, resilience, density, and so on can be varied within wide limits by the choice of each time two components. Polyurethane foams have proved of particular advantage. 7

In this application it is of essential importance that the foamed material is a closed-celled material or, it also open-celled material should be present, that the surface is covered with a sealing layer, for example an epoxy resin lacquer. As a result of this the gas is prevented from penetrating into the irradiated space, so that no corrosion phenomena as a result of gas decomposition can occur. As compared with an insulation by a gas under pressure only, the insulation with foamed material has the advantage that the tube is mechanically damped and supported in a nearly ideal manner by the surrounding body of foamed material.

In order to avoid electric charges it is recommendable in the embodiment described to metallize the surface of the foamed material engaging the housing while, to lengthen the surface leakage paths, the non-metallized surface may be undulated in known manner.

In the embodiment shown in FIGURE 1 there is, of course, no restriction to SP as regards the insulating material; another insulating material, for example, oil, may alternatively be used. As compared with an insulation entirely by oil, the advantage is obtained in this case also that the tube is mounted so as to be insensitive to shocks.

The device shown in FIGURE 1 comprising preformed bodies of foamed material is particularly suitable if housings for insulation with oil or pressurized gas are already present.

In FIGURE 2 the same reference numerals as in FIG- URE 1 denote the same components. In this embodiment the whole free space between the X-ray tube 1 and the housing 5 is filled with foamed material 13, in which by suitable means not shown in FIGURE 2 the exhaust of the air present in the housing is ensured. The tube 1 is embedded in the foamed material 13 in such manner that expensive sockets of cast resin, which, in the case of liquid or gaseous insulating substances, serve for receiving the high-voltage plugs and for securing the tube to the housing, are superfluous. The sockets 4 consist of a particularly hard foamed material. In the cavities 12 of these sockets the high-voltage plugs (not shown) are inserted. In manufacturing sockets 4 it may be ensured that the material is foamed only after the foamed material 13 has solidified in the relative space; the cavities 12 are provided by means of moulds.

As compared with the normal insulation by oil or pressurized gas, the embodiment shown in FIGURE 2 has a series of further advantages; since the foamed material prevents the penetration of air into the space between the housing and the electrodes, it is not necessary to seal the housing. Furthermore the tube is fully protected against mechanical. shocks by the foamed material so that additional means which support the X-ray tube in a resilient manner need not be used. In addition foamed material has the advantage that the relative dielectric constant is approximately 1 so that in the glass of the wall of the tube a smaller electric field strength occurs than, for example, in the case of an insulation by oil. The danger of breakdown of the glass which is sometimes observed in the case of insulation by oil, is consequently avoided. Finally the foamed material serves as a heat insulator which has the favourable effect that the glass bulb assumes a comparatively high temperature and consequently the electric resistance of the glass remains comparatively low and the formation of local charges is avoided.

Finally a metal housing may entirely be omitted as is shown in FIGURE 3. This is possible since the function of the housing as a support of the structural elements conveying the high voltage is taken over by the layers 13 and 14 of foamed material. In this case it is recommendable to make the layer 14 resilient while, as regards the layer 13, it is of particular importance that this layer consists of a material having a closed-celled structure to prevent the penetration of air. For the electric screening and the safeguarding against radiation an earthed lead plate 15 is provided which is arranged between the layers 13 and 14 of foamed material.

In the embodiments shown in FIGURES 1 to 3 the heat produced by the X-ray tubes may be conducted away by cooling devices not shown which may be embedded in the foamed material.

Besides in X-ray apparatus, the invention may be used everywhere where high-voltage conveying parts are arranged in a housing and an insulation with foamed material may be used to advantage because with such a material the problems, for example, the sealing and the mechanical damping, are avoided.

What is claimed is:

1. A high-voltage apparatus comprising an electrical screening member, said apparatus having parts which are at a high potential relative to the screening member, said parts being embedded at least in part in and separated from said screening member by a foamed insulating material the surface of which is sealed to prevent the penetration of gases and liquids thereinto.

2. A high-voltage apparatus as claimed in claim 1, in which the foamed material is polystyrol foam.

3. A high-voltage apparatus as claimed in claim 1, in which the foamed material is phenol resin foam.

4. A high-voltage apparatus as claimed in claim 1, in which the foamed material is polyurethane foam.

5. A high-voltage apparatus as claimed in claim 1, in which the parts at high potential are embedded in preformed bodies of the foamed material.

6. A high-voltage apparatus as claimed in claim 5, in which the remaining free space between the electrical screening member and the high-voltage apparatus is filled with a non-solid insulating material.

7. A high-voltage apparatus as claimed in claim '5, the surfaces of the bodies of foamed material which engage the screening member are metallized.

8. A high-voltage apparatus as claimed in claim 5, in which the surfaces of the body of foamed material are undulated at places which are not in contact with metal parts.

9. An apparatus as claimed in claim 5, in which the surfaces of the bodies of foamed material are provided with a layer of lacquer which prevents the penetration of 10. A high-voltage apparatus as claimed in claim 7, in which the electric screening member is embedded in the foamed material.

11. A high-voltage apparatus as claimed in claim 7, in which the sockets for the voltage supply consists of the foamed material.

12. A high-voltage apparatus as claimed in claim 7, in which the electric screening member consists of a lead plate.

References Cited UNITED STATES PATENTS 2,915,611 12/1959 Luehring 174-50.51 X 3,013,156 12/1961 Hearn 31393 X 3,288,916 11/1966 Koon 174110.8 X

FOREIGN PATENTS 586,376 ll/l959 Canada. 853,941 11/1960 Great Britain.

I JAMES W. LAWRENCE, Primary Examiner E. R. LA ROCHE, Assistant Examiner US. Cl. X.R. 3l355; l7435 

